There is an enormous demand for fluorescent materials nowadays because of their photoluminescence properties which find potential applications in diverse fields, such as chemosensing [1], environmental monitoring [2], biological analysis [3], bioimaging [4-6], organic light emission diodes (OLED) [7-9], and solar cells [10-14]. In the past decade, increasing efforts have been devoted to the development of heavy-metal-free and low-toxicity photoluminescence (PL) nanoparticles in order to overcome the drawbacks of conventional organic dyes (e.g. low water solubility, weak emission in water, and poor photostability) as well as the intrinsic toxicity of inorganic nanoparticles (e.g. quantum dots, named Q-dots) [15,16] and upconversion particles doped with rare earth metals [17]). Various types of organic-based PL nanoparticles have been developed, including semiconducting polymer nanoparticles (P-dots) that consist of π-conjugated polymer [18], organic aggregation-induced emission (AIE) dots assembled from small phenyl containing organic molecules having propellers or pinwheel shapes [19], and photoluminescent carbon nanostructures [20, 21]. Recently, a new class of organic luminescent materials derived from non-conjugated macromolecules has been scarcely reported. For examples, a pure oxygenic non-conjugated macromolecule of poly[(maleic anhydride)-alt-(vinyl acetate)] has been reported to display strong light emission in organic solvents such as THF, NMP, DMSO and DMF [22]. A non-conjugated polyacrylonitrile (PAN) which is almost non-luminescent in dilute solutions becomes highly emissive when concentrated or aggregated as nanosuspensions, solid powders and films [23]. In contrast to conventional fluorescent molecules which suffer from aggregation-caused quenching, the PL property of the non-conjugated polymer is attributed to the aggregation-enhanced emission mechanism such as clustering-triggered emission.
Branched polyethyleneimine (PEI) is a water-soluble polymer that consists of 25% primary, 50% secondary and 25% tertiary amines. This amine-rich PEI (25 kD) has been found to display very weak blue emissions (fluorescence quantum yield Φf˜0.01) in water [24].
Two approaches have been reported to enhance the optical properties of the PEI.
Firstly, crosslinking low molecular weight and branched PEI to form PEI-based nanoparticles. Yang's group recently reported PEI-based photoluminescence dots through crosslinking low molecular weight and branched PEI (Mw=1800) with a carbon tetrachloride (CTC) [25]. The resulting crosslinked PEI particles possess a broad size distribution with an average hydrodynamic diameter of ca. 180 nm measured by dynamic light scattering (DLS). The enhanced PL intensity of the crosslinked PEI may be attributed to the decreased in vibration and rotation of amino-based chromophores, leading to a crosslinking enhanced emission (CEE) effect. They have also demonstrated that this type of water-dispersible PEI-based photoluminescent dots is able to achieve targeted cell imaging [26]. However, this method usually generates particles with a broad size distribution because the crosslinking reaction is difficult to control. Furthermore, the maximum quantum yield of the crosslinked PEI nanoparticles is less than 10%.
Secondly, self-assembly of hydrophobically modified PEI into nanoparticles has been disclosed. Sun et al reported ultra-bright and multicolor PEI-based polymer dots which were fabricated via first conjugation of hydrophobic polylactide (PLA) to PEI (25 kD), followed by generation of the PEI-PLA dots using an emulsion/solvent evaporation technique [27]. A weight ratio of D, L-lactide/PEI of 60 was found to give the copolymer an optimal balance between hydrophobic and hydrophilic segments. The fluorescence quantum yield of the resultant nanoparticles (ca. 227 nm in diameter) was up to 31%, which is 30 times higher than the native PEI in water. Moreover, the emission spectra of the PEI-PLA dots were generally broad and sensitive to the excitation wavelengths (multi-color fluorescence).
Luo et al reported the formation of PEI-based fluorescent nanoparticles through the Schiff base reaction between amines in PEI and formyl group in D-glucose, and then self-assembly of D-glucose conjugated PEI in aqueous solution. The resultant nanoparticles had an average hydrodynamic diameter of ca. 342 nm with surface charge around 11.2 mV. The PEI-based dots exhibit excitation independent emission property. The emission wavelength is centered at 465 nm, and the quantum yield of the nanoparticle is reported as high as 46% using quinine sulfate as a reference [28-29]. However, this approach involves tedious multiple step syntheses of hydrophobically modified PEI copolymer and subsequent self-assembly process through emulsion/solvent evaporation technique. Furthermore, the particle stability is strongly affected by acid because of easy hydrolysis of the imine or ester linkage. Finally, the self-assembly process is not amendable for a scale-up production.